1. Field of the Invention
The present invention relates generally to a transmission controlling method in a mobile communication system, and in particular, to a method of controlling reverse transmission.
2. Description of the Related Art
With the phenomenal growth of mobile communication technology, many different mobile communication systems have been proposed and are currently being field-tested. These systems generally operate based on CDMA (Code Division Multiple Access) and a 1xEV-DO (1x Evolution-Data Only) system called HDR (High Data Rate), designed to carry out high-rate data transmission dedicatedly.
Similarly to other systems, 1xEV-DO systems also require appropriate scheduling to efficiently transmit packet data on the forward and reverse links. “The forward link” is a link directed from a base station to an access terminal (AT) and “a reverse link” is the opposite link. For forward data transmission, the base station transmits data to a particular AT in the best channel condition, considering the air link statuses between the base station and 1xEV-DO ATs and other environments, resulting in a maximized data transmission throughput for the AT. Concerning reverse data transmission, a plurality of ATs access the base station simultaneously. In this situation, the base station controls overload within reverse link capacity by appropriately controlling congestion and data flows from the ATs.
Besides the 1xEV-DO systems, other mobile communication systems designed to support multimedia service must also manage reverse data transmission efficiently. To do so, system performance and capacity must be ensured.
In the existing 1xEV-DO systems, an AT carries out reverse data transmission based on an RAB (Reverse Activity Bit) and an RRL (ReverseRateLimit) message received from a base station, and tells the base station its variable data rate via an RRI (Reverse Rate Indicator). The RRI indicates to the base station the data rate at which the reverse traffic data is being sent. The base station transmits the following time-multiplexed channels to the AT: a forward MAC (Medium Access Control) channel, a pilot channel, an FAB (Forward Activity Bit) channel and an RAB channel. The RAB represents the congestion degree of the reverse link and a data rate available to the AT varies according to the RAB. The base station controls a data flow from the AT by commanding an increase/decrease in the reverse data rate using the RAB to control the overload and capacity of the reverse link. Since the RAB is broadcast to a plurality of ATs, the ATs receiving the RAB increase their data rates or reduce them uniformly according to the RAB. The transmission time (or transmission period) of the RAB is determined by Equation (1):T mod RABlength  (1)to where T is system time and RABlength is the length of the RAB expressed in the number of slots. Table 1 below lists binary values representing RAB lengths. The base station transmits one of the binary values to the ATs and then the ATs calculate a slot time they receive the RAB on a forward MAC channel (F-MAC channel) using the received RABlength information and the system time.
TABLE 1BinaryLength (slots)008011610321164
An AT receives a persistence vector defined in a message from the base station at or during a connection. When RAB=0, the persistence vector is set to increase the reverse data rate and when RAB=1, it is set to decrease the reverse data rate. Using the persistence vector, the AT performs a persistence test. If the persistence test is passed, the AT will either increase the reverse data rate or reduce it. If the persistence test is failed, then the AT maintains the reverse data rate.
To describe in detail, if the RAB is 0 and the persistence test is passed, the reverse data rate is increased. On the contrary, if the RAB is 1 and the persistence is passed, the reverse data rate is reduced. The success or failure of the persistence test depends on whether a generated random number satisfies a desired condition. Since the reverse data rate varies with uncertainty in probability terms, the base station does not know how many ATs increase/decrease their data rates. Time delay occurs on the reverse link until a high capacity state is transitioned to a full utilization state. In the reverse link full utilization state, overload and underload alternate. However, the base station cannot know how serious the overload or underload condition because the base station simply transmits an RAB and the ATs increase/decrease their data rates according to the results of persistence tests.
If the overload condition becomes serious, it leads to the increase of reverse data loss. On the other hand, if the underload condition becomes serious, the use efficiency of reverse channels is dropped. Therefore, there is a need for exploring a method of rapidly reaching the reverse link full utilization state and a method of increasing the use efficiency of reverse channels, while preventing the occurrence of overload in the base station.
FIG. 1 is a flowchart illustrating a reverse data rate controlling procedure for an AT in an existing 1xEV-DO system.
The AT sets its lowest available data rate at an initial reverse data transmission. If the current data rate is lower than a data rate provided in an RRL message received from a base station, the AT transmits data at the provided data rate after 32 slots (53.33 ms). On the other hand, if the current data rate is higher than the provided data rate, the AT transmits data at the provided data rate. For the subsequent reverse transmission, the AT determines its data rate by the procedure illustrated in FIG. 1. The RRL message is transmitted to the AT in determining an initial reverse data rate and resetting the reverse data rate.
After determining its data rate, the AT reports its data rate to the base station by an RRI symbol as shown in Table 2. The reverse data rate is selected among 4.8, 9.6, 19.2, 38.4, 76.8 and 153.6 kbps. This reverse data rate is reset by a message such as an RRL message or an RAB message received from the base station. Table 2 below lists RRI mappings in the 1xEV-DO system.
TABLE 2Data rate (kbps)RRI symbol4.80019.601019.201138.410076.8101153.6110
The base station determines the data rate of the AT from the RRI symbols as shown in Table 2 and controls the AT to reset its data rate. To aid the AT in resetting its data rate, the base station should transmit an RRL message as shown in Table 3 to the AT.
TABLE 3FieldLength (bits)Message ID829 occurrences of the following two fieldsRateLimitIncluded1RateLimit0 or 4ReservedVariable
The RRL message is forward directed to control a reverse data rate. Upon receipt of the RRL message, the AT resets the reverse data rate by comparing the current reverse data rate with a data rate set in the RRL message. 29 records may be inserted in the above RRL message and each record indicates a data rate assigned to a corresponding MACindex among MACindexes 3 to 31. In Table 3, Message ID indicates the ID of the RRL message. RateLimitIncluded is a field indicating whether RateLimit is included in the RRL message. If RateLimit is included, RateLimitIncluded is set to 1 and otherwise, it is set to 0. RateLimit indicates a data rate assigned to a corresponding AT. The base station assigns data rates shown in Table 4 to ATs using four bits.
TABLE 40x0 4.8 kbps0x1 9.6 kbps0x219.2 kbps0x338.4 kbps0x476.8 kbps0x0153.6 kbps All other values are invalid
During reverse data transmission, the AT monitors a F-MAC channel from the base station, especially the RAB on the F-MAC channel, and adjusts its current data rate by performing a persistence test.
Referring to FIG. 1, the AT monitors the RAB of an F-MAC channel from a base station included in the active set of the AT in step 100 and determines whether the RAB is 1 in step 102. If the AT has six sectors/base stations in its active set, it determines whether at least one of the RABs of the F-MAC channels received from the six sectors/base stations is 1. If at least one RAB is 1, the AT proceeds to step 112 and otherwise, it goes to step 104.
The case where all RABs=0 will be considered first.
If the RAB is 0, the AT performs a persistence test in step 104. The persistence test is available when the base station broadcasts the RAB to a plurality of ATs to control the amount of reverse data from the ATs. The persistence test is passed or failed depending on whether a generated random number satisfies a desired condition.
If the persistence test is passed in step 104, the AT increases its data rate (TX rate) in step 106. On the contrary, if the persistence test is failed, the AT jumps to step 120. The AT increases the TX rate in step 106 and compares the increased TX rate with a maximum allowed data rate (a max TX rate) in step 108. If the increased TX rate is higher than the max TX rate, the AT sets the TX rate to the max TX rate in step 110 and goes to step 120. If, in step 108, the increased TX rate is not higher than the max TX rate, the AT goes directly to step 120
Now, the case where at least one RAB=1 will be considered.
If the RAB is 1 in step 102, the AT performs a persistence test in step 112. If the persistence test is failed, the AT jumps to step 120. If the persistence test is passed, the AT decreases the TX rate in step 114 and compares the decreased TX rate with a minimum data rate (a min TX rate) in step 116. If the decreased TX rate is lower than the min TX rate, the AT goes to step 118 and otherwise, it jumps to step 120. The AT sets the TX rate to the min TX rate in step 118 and goes to step 120. The min TX rate can be a default data rate of 9.6 kbps or a data rate designated by some message at a call connection.
In step 120, the AT generates an RRI symbol corresponding to the set TX rate. The AT transmits the RRI symbol along with traffic data only if a traffic connection is opened between the base station and the AT. If the traffic connection is not opened, it transmits only the RRI symbol.
FIG. 2 is a diagram illustrating data transmission/reception between an AT and an HDR sector included in the active set of the AT. As illustrated in FIG. 2, F- and R-traffic channels, and F- and R-MAC channels have been established between the AT and sector 1 with a connection opened between them. No F-traffic channels are assigned to the AT from sector 2 (up to sectors 2 to 6) with no connection opened between them. In the 1xEV-DO system, the AT can maintain up to six sectors/base stations in its active set. Therefore, to determine its TX rate, the AT monitors F-MAC channels from all the sectors of the active set, especially RABs on the F-MAC channels.
Upon receipt of at least one RAB set to 1, the AT performs a persistence test to decrease its TX rate. In the persistence test, the AT generates a random number and compares it with a persistence vector defined by the base station at or during a connection. If the random number satisfies a desired condition, the AT determines that the persistence test is passed. The AT then decreases the TX rate. On the contrary, if the persistence test is failed, the AT maintains the TX rate. If the TX rate is lower than a min TX rate, the AT sets the TX rate at the min TX rate. Meanwhile, if all the RABs are set to 0 and a persistence test is passed, then the TX rate is increased. If the persistence test is failed, the AT maintains the TX rate. If the TX rate becomes higher than a max TX rate, the AT sets the TX rate to the max TX rate. Also, when the AT is limited in transmission power, it maintains the TX rate. The RAB that increases or reduces a reverse data rate is broadcast to ATs in TDM with an FAB on a forward common channel, i.e., a F-MAC channel. The ATs increase/decrease their data rates uniformly according to the RAB.
From the system's perspective, the above-described reverse transmission controlling method for the current 1xEV-DO systems simplifies bandwidth control and overhead control. However, the uniform control without considering the individual statuses of ATs brings about a bandwidth waste and decreases the data transmission efficiency of the ATs.
Moreover, a long time delay is involved in reaching a full utilization state on the reverse link, resulting in the decrease of channel use efficiency. The occurrence of an overload may lead to reverse data loss. As a result, communication quality is deteriorated.